Communication devices such as mobile stations are also known as e.g. mobile terminals, wireless terminals and/or User Equipments (UEs). Mobile stations are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system. The communication may be performed e.g. between two mobile stations, between a mobile station and a regular telephone and/or between a mobile station and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Mobile stations may further be referred to as mobile telephones, cellular telephones, or laptops with wireless capability, just to mention some further examples. The mobile stations in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another mobile station or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the mobile stations within range of the base stations.
In some radio access networks, several base stations may be connected, e.g. by landlines or microwave, to a radio network controller, e.g. a Radio Network Controller (RNC) in Universal Mobile Telecommunications System (UMTS), and/or to each other. The radio network controller, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural base stations connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for mobile stations. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
According to 3GPP/GERAN, a mobile station has a multi-slot class, which determines the maximum transfer rate in the uplink and downlink direction. GERAN is an abbreviation for GSM EDGE Radio Access Network. EDGE is further an abbreviation for Enhanced Data rates for GSM Evolution.
In the context of this disclosure, the expression DownLink (DL) is used for the transmission path from the base station to the mobile station. The expression UpLink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
A vision of a development of the communication in cellular networks comprises huge numbers of small autonomous devices, which typically, more or less infrequently, e.g. once per week to once per minute, transmit and receive only small amounts of data. These devices are assumed not to be associated with humans, but are rather sensors or actuators of different kinds, which communicate with application servers, which configure the devices and receive data from them, within or outside the cellular network. Hence, this type of communication is often referred to as machine-to-machine (M2M) communication and the devices may be denoted machine devices (MDs). In the 3GPP standardization the corresponding alternative terms are machine type communication (MTC) and machine type communication devices (MTC devices). Note that in a wider context an MTC device is just another type of User Equipment, albeit with certain special characteristics.
With the nature of MTC devices and their assumed typical uses follow that they will often have to be very power efficient, since external power supplies will often not be available and since it is neither practically nor economically feasible to frequently replace their batteries.
Regarding MTC devices, the M2M activity in future radio access development is considering an alternative access mode, based on contention. A generic term for such an access mode is Contention-Based Protocol (CBP). A CBP is a communications protocol for operating wireless or wireline telecommunication equipment that allows many users to use the same radio channel, or wire or other physical resource, without pre-coordination. One such access mode that has been discussed in 3GPP is often referred to as Contention Based (CB) access. It allows any UL synchronized UE to transmit UE data using a certain UL radio resource without having received a prior dedicated allocation of the resource. A UE with UL synchronization knows in what time instants it may transmit in the UL in order for its transmissions to arrive at the base station in a manner that is synchronized with other UL transmissions from other UEs, so that the UL transmissions from different UEs which are intended to arrive at the base station in sequence to not overlap each other in time and thus do not interfere with each other. Closely related to the concept of UL synchronization is the term timing advance (TA) which defines a relation between DL and UL transmissions in a UE and which is used by a UE to derive the correct UL transmission time instants. The timing advance is the time difference between a UE's transmission in the UL and the start of a transmission resource, e.g. a subframe in LTE, in the DL as perceived by the UE. That is, a UE transmits UL data slightly before the UL transmission resource (e.g. LTE subframe) boundary as perceived from the DL receptions. Thus, in essence, timing advance is a negative offset, at the UE, between the start of a received DL resource, e.g. LTE subframe, and an UL transmission, e.g. a transmitted UL subframe in LTE. This negative offset is used to compensate for the propagation delay on the distance from the radio base station to the UE and back again. The radio base station monitors the timing misalignments in its receptions from UEs and sends timing advance commands, e.g. in a Medium Access Control (MAC) control element in a MAC Protocol Data Unit in LTE, in the form of relative changes, to UEs when needed.
In WO2010/057540, which discloses the CB access mode, access for unsynchronized user equipments is mentioned. To cope with the transmission and reception timing misalignment resulting from the lack of UL synchronization it is disclosed to reserve the resource blocks following the CB resource blocks and not schedule any transmission in these resource blocks. A resource block in LTE terminology is a set of resource elements, where a resource element is a small entity in a time-frequency grid, i.e. a transmission resource consisting of a certain bandwidth in the radio frequency dimension and a certain length of time in the time dimension and wherein one resource block carries one Orthogonal Frequency Division Multiplexing (OFDM) symbol. An OFDM symbol is a radio modulation symbol, i.e. the smallest information carrying unit in the radio communication, representing one or several data bits depending on the applied modulation scheme. In LTE the bandwidth of a resource element is 15 kHz and a resource block consists of 12 “sub-carriers” with a bandwidth of 15 kHz each, yielding a total resource block bandwidth of 180 kHz. In the time dimension an LTE resource block is 0.5 ms and typically comprises 7 resource elements (and thus 7 OFDM symbols) on each sub-carrier. This results in a total of 12×7=84 resource elements (and thus 84 OFDM symbols) per resource block in LTE. In the context of this document the term “Opportunistic Transmission Mode” (OTM) is used to denote any CBP, which would allow any machine device, with or without UL synchronization, to transmit using an allocated UL resource.
MTC devices without UL synchronization will not be able to transmit with enough timing accuracy, they require larger guard times than synchronized MTC devices and/or user equipments. A guard time is a time margin used after a transmission to ensure that the transmission does not overlap and interfere with a later transmission from another UE using the same frequency or frequencies. This is a problem since existing transport formats are inherently adapted to the size of the possible resource allocations. Providing guard times in the form of, and with the granularity of, entire resource blocks is however wasteful in terms of system resources and results in resource inefficient transmissions.
A further complication is that cells vary a lot in size, sometimes in ways not predicted at the cell planning. Hence, the maximum guard time requirements will vary with different cell sizes. The reason for this is that the required guard time depends on the distance between the UE and the base station, as its purpose is to provide sufficient margin for an otherwise uncompensated, as an unsynchronized UE is assumed, distance-dependent propagation delay. A solution that provides sufficient guard time margins for large cells will result in unnecessarily large guard times when used in small cells, which in turn means that OTM/CB resources are inefficiently used and system resources are wasted.
A very large cell, e.g. 100 km radius has a cell edge to radio base station roundtrip propagation delay of 0.67 ms, corresponding to eight extended Orthogonal Frequency Division Multiplexing (OFDM) symbols in LTE, wherein an OFDM symbol is a radio modulation symbol, i.e. the smallest information carrying unit in the radio communication, representing one or several data bits depending on the applied modulation scheme. Propagation delay is the time it takes for radio transmission, i.e. electromagnetic radiation, to propagate a certain distance.
As a comparison, the largest guard time from the different specified LTE Random Access preamble formats is 0.72 ms, corresponding to almost nine extended OFDM symbols.
Thus, if the propagation delay from a UE without UL synchronization is not compensated for, e.g. by an increased guard time, it may interfere with significant parts (e.g. several OFDM symbols) of a transmission from another UE which were intended to arrive at the base station after the transmission from the UE without UL synchronization. This may ruin at least part of the reception of the transmissions from both UEs, causing decreased transmission quality and/or retransmissions, increased power consumption, increased delays and poor resource utilization.